An NMR study of the nitration of toluene over zeolites by HNO3-Ac2O

Liquid and solid state NMR studies of the nitration of toluene with a mixtu re of nitric acid and acetic anhydride have been performed in homogeneous s olution and in a heterogeneous system using zeolite beta (small and large c rystalline Al-beta with Si/Al = 13 and pure silicon beta), mordenite (Si/Al = 74 and 5) and ZSM-5 (Si/Al = 20) as catalysts. Acetyl nitrate proved to be the reactive species, although it decomposed slowly to tetranitromethane which was not active in the nitration. No NMR evidence for the presence of isolated nitronium ions, often thought to be the nitrating species, was fo und in the liquid phase. In the absence of acetic anhydride, nitric acid in teracted strongly with the framework aluminium of the zeolite leading to a complex of one nitrate per aluminium. In contrast to free nitric acid, this species did not seem to take part in the nitration of toluene. Introductio n of acetic anhydride converted the strongly bonded nitrate to the more rea ctive acetyl nitrate and acetic acid which were coordinated to the framewor k aluminium of the zeolite. Heterogeneous zeolite-catalysed nitration of to luene by acetyl nitrate improved the para- to ortho-nitrotoluene ratio comp ared to the homogeneous liquid phase nitration. The best para-selectivity w as obtained with Al-beta and to a lesser extent with dealuminated mordenite , suggesting that the selectivity may be related to the presence of flexibl e lattice aluminium in these large pore zeolites.